214 Mr. J. Gill on the Dynamical Theory of Heat. 



loss of heat; and secondly, by free tumultuous expansion from 

 the containing vessel into an annexed vacuous receiver, by which 

 no exterior work was performed, and no loss of heat was observed. 

 It was deduced that heat disappeared in the first case, being 

 converted into the equivalent work produced ; and that in the 

 second case no heat disappeared because no work was done. 

 These striking experiments drew the attention of scientific men 

 to the subject of the new theory ; and Regnault in particular 

 interested himself in repeating them with the variety and accu- 

 racy suggested by his great experience in thermic researches. 

 He announced to the Academy of Sciences ten years ago that his 

 experiments fully confirmed Joule's results, and from that time 

 he became an open partisan of the new theory 



The facts of these experiments are correct (indeed they could 

 hardly have been otherwise in the hands of such able experi- 

 menters) ; but I submit that the explanation given to them is 

 incorrect. The compressed air was of the high tension of twenty- 

 two atmospheres, and to produce this tension no small amount of 

 work must have been expended. The energy equivalent to this 

 work was certainly taken from some exterior source and transferred 

 to the air. If no heat escaped during the compression, the tem- 

 perature would rise very considerably, from the change of latent 

 heat to sensible ; and when the air was allowed to expand back 

 to its former volume under moderated resistance, an amount of 

 work would be produced equal to that expended on the compres- 

 sion, the temperature falling to its original point by a changing 

 back of sensible heat to latent. If the escape of heat is not pre- 

 vented during compression, the dense air, at the temperature of 

 the surrounding atmosphere, will contain much less heat than it 

 held at the initial density with the same temperature, and its 

 tension will be proportionately less than if the heat were retained. 

 If no heat be allowed to re-enter the air-vessel during the expan- 

 sion, the temperature will fall very considerably, and less work 

 will be produced by the smaller amount of expansion which, 

 under these circumstances, will restore the equilibrium. In the 

 former case all the energy put into the compressed fluid remains 

 in it as statical energy, or packed-up force, while the state of 

 compression lasts ; and in the latter case, part of the communi- 

 cated energy escapes with the heat, and the rest remains stored 

 up in the compressed fluid. This latter state of things repre- 

 sents the initial condition of the compressed air in Joule's expe- 

 riments; and it is evident that if the air be allowed to expand 

 by gradually enlarging the space which contains it, exterior work 

 will be produced, and the temperature of the expanding fluid will 

 fall by a change of part of its sensible heat into latent, the whole 

 quantity of heat remaining the same notwithstanding the fall of 

 temperature. 



